FIG. 1 is a simplified block diagram of a communication system transmitter, in which a data source 12 is coupled to a frequency-hopping modulator 14, which simultaneously frequency hops at a rapid rate, and modulates the data onto the hopping carrier, as by amplitude or phase modulation, for example. The hopping rate may be equal to the data rate, or it may differ. One possible hopping rate is ten kilohops/second. The modulated carrier is applied over a path 17 to a phased-array antenna 18. Phased-array antenna 18, as known, transmits the signal power into space in one or more beams, under the control of phase-shifter control signals applied thereto over a path 22 from a phase-shifter controller 20.
FIG. 2 is a simplified diagram illustrating a prior-art phased-array antenna which may be used in the system of FIG. 1, as so far described. In FIG. 2, a line array of elemental antennas 210a, 210b, 210c, . . . 210n is fed with RF signals from an array of individual controllable phase shifters 212a, 212b, 212c, . . . 212n, the phase shifts of which are individually controlled by phase shifter control signals applied over a bus 22. The elemental antenna elements are collectively designated 210, and the phase-shifters are collectively designated 212. Each phase shifter 212a, 212b, 212c, . . . 212n, in turn, is fed with RF from a single port or path 17. Those skilled in the art know that the phase shifters of FIG. 2 are controlled to produce a planar wavefront, such as 214, which in turn results in a beam, conventionally illustrated as beam 216, directed in a direction normal to or orthogonal to the planar wavefront 214. The preceding discussion is valid for single-frequency operation, or operation over a narrow band of frequencies. However, when the frequency of operation varies over a significant range, another effect occurs. The phase-shift required to achieve a planar phase front changes with frequency, so that the phase shift at a first or base frequency of operation may be selected to provide the desired planar wavefront direction and resulting beam direction, but will change as the frequency is deviated away from the base frequency. In FIG. 2, the effect of a decrease in frequency, which decreases the required phase-shift imparted by the phase-shifters, is illustrated by a planar wavefront 218, and the change in beam direction is illustrated by beam 220. The offset or "squint" angle due to the frequency change is illustrated as .theta.. The squint problem can be solved by the use of controllable delays instead of phase shifters in the arrangement of FIG. 2, because the amount of delay does not vary with frequency in an ordinary delay line. However, delay lines, and especially controllable delay lines suitable for high-power applications, tend to be heavy, bulky, and expensive. Consequently, phase shifters are preferred.
It is possible arrange phase control 20 of FIG. 1 to readjust the phase shifters 212a-212n of the phased-array antenna of FIG. 2 each time the frequency is changed. The calculations required to determine the phase shift required for each phase shifter are not trivial, however, so ultrafast controllers may be required, depending upon the rate of frequency hopping, which controllers are capable of performing the calculations within the time allowed for the frequency hop. As an alternative, a plurality of predetermined phase values can be stored in memory, with the phase control value for each phase shifter at each frequency and each beam angle stored in memory, and accessed for control of the phase shifters. This arrangement is disadvantageous because it requires substantial memory capacity for each phase shifter if a significant number of frequencies and beam directions are to be available. If small memories are used, the number of beam directions and frequencies of operation will likewise be limited.
Improved frequency-hopping phased-array systems are desired.